Many modern buildings have large windows used for better lighting of the interior. These windows, made of glass, let through the visible as well as a large part of the infrared solar light into the building, which affects the temperature inside the building. Although the infrared radiation helps to reduce heating expenses during winter, it adds more heat that needs to be removed by air conditioning in summer times. Solar light (or sunlight) includes UV light (including UVC, UVB, and UVA light), visible light, and infrared light. From an energy saving point of view, it would be wise to reflect the infrared light in summer times and to absorb the infrared light in winter.
In the past, there have been attempts to realize smart passive windows that are infrared reflecting. Conventional systems do not teach a window-filter that has both absorbing properties and that can reflect the infrared light in summer (warmer) temperatures and, when turned around, absorb the infrared light in winter (colder) temperatures.
Energy saving, passive, reversible window-filters that are coated with alternating thin metallic and dielectric layers that transmit most of the visible light while reflecting most of the light in the infrared part of the spectrum when impinged by solar light on one side and transmit most of the visible light while absorbing most of the light in the infrared part of the spectrum when impinged by solar light on the other side are a real need.